makefiles/murdock.inc.mk: do not overwrite FLASHFILE if set

If FLASHFILE is set keep the original value.

It changes the variable from an immediate to a deferred variable but if
murdocks keeps working there is no issue.

makefiles/murdock.inc.mk

@@ -2,8 +2,8 @@
 # This file contains helper targets used by the CI.
 #
 
-# (HACK) get actual flash binary from FFLAGS.
-FLASHFILE:=$(filter $(HEXFILE) $(ELFFILE:.elf=.bin) $(ELFFILE),$(FFLAGS))
+# (HACK) get actual flash binary from FFLAGS if not defined.
+FLASHFILE ?= $(filter $(HEXFILE) $(ELFFILE:.elf=.bin) $(ELFFILE),$(FFLAGS))
 
 #
 # This target will run "make test" on the CI cluster.